1. Field of the Invention
The present invention relates, generally, to composite materials, and methods of making and using composite materials. More particularly, the invention relates to composite materials which are hardened and maintained under self-compression.
2. Background Information.
Common conventional fiber reinforced composites contain fibers such as glass, polymeric and carbon fibers in either continuous or chopped forms. The matrix materials are plastic resins such as the polymerization product of unsaturated polyester with styrene or the reaction product of amines with epoxy resin. The matrix material may also be a thermoplastic. Many other variations are known. Conventional composites are strong, impact resistant and lightweight. These properties are useful in the construction industry. In U.S. Pat. No. 4,316,925 carbon fiber composites are used to reinforce concrete. In some cases the fibers are tensioned to form the composite and align the fibers. For example in U.S. Pat. No. 5,114,653 polymeric fibers are used with an epoxy matrix to provide prestressed reinforcing rods for concrete. In U.S. Pat. No. 4,063,838 tensioned glass fiber composites are used to replace paint as a protective barrier for metal. In U.S. Pat. No. 5,362,545 wood is similarly protected.
Although a large number of conventional composites have been disclosed, the use of conventional composites in construction has been very limited. The reason is related to the costs of these materials relative to common construction materials such coated steel reinforcing rods, wood beams and molded or extruded plastic pipe, drains and the like. Also, conventional composites exhibit some serious deficiencies in their mechanical and chemical properties.
Construction materials usually need to be strong, rigid and inexpensive. Conventional composites usually have good strength but have limited rigidity. The rigidity is reflected in a material's modulus. The modulus of typical conventional composites is about 20 GPa (gigapascals) to 75 GPa, although some expensive exceptions exist. Steel, by comparison, has a modulus of about 210 GPa. Therefore it is often found that in order to obtain rigidity comparable to common materials the conventional composite must be used to an extent that the cost becomes one to two or more orders of magnitude, or more, higher than that of the common materials. An additional contributing factor is that although selected fibers may have a high modulus, the matrix resins have generally low moduli, typically from about 2 GPa to 5 GPa. The combination of fibers plus resin has substantially lower modulus than the fibers alone. Composites with costly very high fiber volume fractions thus need to be used to retain rigidity.
Another mechanical limitation of conventional composites is their failure mode. Steel, when highly stressed, exhibits ductile failure; thus steel continues to be load carrying at high stresses. Conventional composites usually exhibit brittle failure, therefore the load of the failed member is rapidly transferred to the remaining load carrying elements in the structure. This can lead to catastrophic failure.
Conventional composites also have limitations due to their chemical nature. Epoxy resins are photochemically reactive and thus need to be protected from outdoor light exposure. They also can be water sensitive. Polyester and vinyl ester resins, being esters, can be hydrolyzed in high pH environments, such as when embedded in concrete. Conventional composites, when used as coatings or barriers for common materials, also exhibit many of the problems of ordinary paint. Moisture can be trapped at the interface between the conventional composite and the common material and can cause blistering and corrosion. Gasses, such as hydrogen, can also cause bond failure at interfaces with steel.
To overcome the limitations of conventional composites, a large number of cementitious composites have been disclosed. Cementitious composites contain fibers and inorganic cement matrix materials such as Portland cement. More generally, inorganic cements have been classified as hydraulic cements, such as Portland cements, alumina cements, natural cements, pozzolans and slag cements and non-hydraulic cements such as gypsum and magnesium cement and the like. Hydraulic cements are water resistant when hardened. Some additional cements, not strictly belonging to these two groups are lime cement and various other silicate based cements. Cements usually contain water and chemical admixtures and often contain polymers and inert materials such as crystalline silica or other minerals. The corresponding composites belong to a class of composites called brittle matrix composites, because the neat matrix materials will exhibit fracture at very low strains, typically less than 0.1 percent. Other brittle materials include ceramics, glassy materials and some metals.
The cementitious composites, which have greatly improved ductility over the neat matrix, contain fiber volume fractions between 10 and about 30 percent. Volume fractions below about five volume percent may be useful for applications such as crack control, but they do not provide a marked improvement in strength and ductility over the neat matrix. Fiber volume fractions above about 30 volume percent are possible but they are more difficult to achieve due to the problem of mixing fibers with particulate containing matrix materials without excessive void formation; i.e. the problem of consolidation. By reference, the volume fractions of fibers in conventional composites is usually between about 10 and 70 volume percent.
Cementitious composites offer potential solutions to the limitations of conventional composites. The cements can be lower cost, often by an order of magnitude, than the resins of conventional composites. Further, cements usually have a very high moduli, typically from about 10 GPa to over 40 GPa. This offers the possibility of formulating composites with sufficient amounts of expensive materials such as fibers, to provide the required strength and ductility, while using much more of the low cost matrix material to provide the rigidity. That is, the structural element can be made as rigid as required, while retaining low cost, by making the element larger, as long as the fiber volume fraction remains high enough to provide good composite properties.
Cements have better weathering properties than resins since they are usually much less sensitive to light and water. Cementitious composites also can provide good barrier properties to common construction materials. The high pH of some cements can protect metals by passivation while the cementitious composite can provide strength and impact and abrasion resistance that is far superior to paint films. The cementitious composites usually are not complete barriers, thus they allow the slow passage of water and gasses which prevents damage to the interface of the composite and the surface of the material to be protected and/or reinforced, such as metals or treated wood. Cementitious composites can also provide protection and rigidity to certain plastic elements, such as polyolefin pipe and the like at low cost.
However, cementitious composites are severely limited by the low strength of the cements and by their brittle nature. Cements typically have low strength in compression and have extremely low strength in tension relative to matrix resins.
Another limitation is that it is difficult to mix fibers and cement pastes to obtain uniformity. It is much less difficult, for example, to saturate fibers with liquid resin. To reduce the mixing problem and to improve strength, short fiber cementitious mixes have often been made with excess water, then partially dewatered before use by a process of water removal related to filtration. Further improvements have been made by also including continuous fibers in the short fiber composite. In U.S. Pat. No. 4,077,577 short fibers of asbestos are used to reinforce a cementitious matrix for a pipe. The pipe also is reinforced with helically wound continuous aramid fibers. The short fiber cementitious matrix thus becomes the matrix for the continuous fibers. The short fiber matrix is made by dewatering. However, the improvement of U.S. Pat. No. 4,077,577 is costly to implement and requires toxic materials.
In U.S. Pat. No. 4,810,552 a sheet shaped article is disclosed, which contains continuous fibers in a cementitious matrix. The continuous fibers are positioned in the plane of the sheet and coated with short fibers in a cementitious matrix. However the matrix remains weak and the short fibers provide very little ductility.
Another technique that has been used to improve the ductility of cementitious materials is represented by a process called SIFCON, or slurry infiltrated fiber reinforced concrete. Here three dimensional loose arrays of short fibers are infiltrated with cement paste, usually with the help of vibration. By providing a three dimensional array of reinforcing fibers, the effect of the brittle matrix is somewhat reduced. However, slurry infiltration is a slow and difficult process. SIFCON materials were studied in the 1970's and were evaluated for missile silos in the early 1980's. SIFCON has been used with conventional steel rod reinforcement in U.S. Pat. No. 4,979,992 to provide structures with improved ductility. However this process is more difficult than simple SIFCON, due to the interference of the rods during cement infiltration.
Long or continuous fibers have been used in a process called SIMCON, which is slurry infiltrated continuous fiber mat reinforced concrete. The mat systems are disclosed in U.S. Pat. No. 3,637,457, U.S. Pat. No. 4,414,262 and U.S. Pat. No. 4,617,219, where three dimensional polymeric nonwovens are used and in U.S. Pat. No. 5,571,628 where shaped mats of long steel fibers are disclosed. The slurry infiltrated mats generally provide the most improved mechanical properties of the composites in the brittle matrix composite group. The improved tensile and flexural properties result from the use of long or continuous fibers. The definition of continuous fibers depends somewhat on the specific material sets, but they can generally be defined by the aspect ratio of the fibers. The aspect ratio is the ratio of the fiber's average length to the fiber's average diameter. Fibers with aspect ratios over 500 generally act as continuous fibers while fibers with aspect ratios below about 200 exhibit composite properties that are greatly affected by fiber end effects and thus act as short fibers in composites. Brittle matrix composites are unusually sensitive to short fibers because the fiber-matrix bond is often weak and the matrix deforms by cracking, thus further weakening the bond.
However, all of the above brittle matrix composites suffer from reduced fiber efficiency; that is, much of the fiber volume fraction is used to reinforce the matrix, rather than be aligned in the directions of expected stress when the composite is used. For example, in a composite with an elongate shape it is desirable to have the lengthwise fibers uniformly aligned so that they can act together to resist tension or bending forces. This has not been possible with brittle matrix composites because of the nature of the response of the matrixes to deformations. When a conventional resin matrix composite is deformed, the matrix is strained by elastic and plastic mechanisms. When a brittle matrix composite is significantly deformed, the matrix strain is produced by the cracking of the matrix. Matrix materials in composites provide connectivity between the fibers so that the fibers can act together to respond to loads. Brittle matrix composites of aligned fibers are damaged when deformed because the fibers can no longer act together due to matrix cracking. The mat approach improves the brittle matrix composite response because the randomly placed continuous fibers bridge the cracks, but at the expense of fiber efficiency. Conventional composites, by comparison, are very useful when the fibers are aligned, such as when used in a uniaxial composite as an aircraft wing spar cap.
Thus the above disclosed prior art provides some routes to improved properties of brittle matrix composites, but fails to provide properties approaching those of conventional composites and thus are not widely used. An additional reason for this failure is the effect of fiber addition when the brittle matrix is a cementitious matrix. Surprisingly, when fibers are added to cementitious materials they usually add to the pore structure. The increased pore content, which presumably is the result of shrinkage and paste segregation, further degrades the properties of the composite. Thus, the addition of fibers causes a weakening effect, which offsets the desired strengthening effect. This issue is described in Alwan J. and Naaman A., Journal of Engineering Mechanics, Vol. 120, No. 11, November, 1994, pg. 2455.
In order to enhance the properties of cement, an improvement can be made by reducing the defect structure during hardening. In U.S. Pat. No. 4,529,567 concrete articles are made by holding the concrete mix under high pressure (at least 50 MPa) during the hardening process. Substantial improvements in mechanical properties are claimed. However, the process of obtaining and retaining hydraulic pressure during the hardening period is difficult in a production process. This can be particularly difficult when hardening shrinkage is considerable.
In U.S. Pat. No. 4,363,667 another approach to reducing defect structure and improving ductility is described. Cement pastes containing small particles are used. Small particles, such as microsilica, fill the voids between the larger cement particles. Small particles can be further effective if they are pozzolanic. The use of microsilica alone increases brittleness, but to provide ductility, the paste is also reinforced with short fibers and mixed with water dispersible polymers. The polymer aids in dispersion, consolidation and strengthening of the composite. Water reducing admixtures are also used to reduce pore content. This technology was later also the subject of academic research, where mixtures were consolidated on two roll mills and thereafter hardened while being molded in a press. Very ductile products were obtained, but these processes were complex and it was difficult to use continuous fibers. Continuous fibers generally provide much improved properties over short fibers in composites.
The effects of application of pressure during hardening on the mechanical properties of hardened cementitious compositions are related to the reduction of voids or flaws. Voids in cementitious systems can be formed during the hardening process due to shrinkage over long time periods. In Neville, A. M., Properties of Concrete, Forth Edition, Wily, New York, 1996, pg. 435, shrinkage is described as continuing at a rapid rate for three months to one year after casting. When pastes are used which contain high concentrations of cement, such as used in SIMCON, the magnitude of the shrinkage can be very large. Therefore the application of pressure for long time periods would seem to be required in order to improve the mechanical properties of cementitious composites by consolidation. This is usually impractical in an industrial operation.
Thus there is a need to provide an improved composite which fulfills the potential offered bv high modulus, brittle matrix materials over conventional resin matrix materials.